Engine in cold start extremely undertouring

Hello everybody, with my MX-5 NA 1.6l the speed is always extremely low at the cold start. It is 150-200 revolutions. Only when it is significantly warmer the speed swings back into the normal range of 850… Although, the speed is then more at 1000-1100… The vehicle then buzzes extremely, but it drives quite normally. Only in a standing position is the whole thing extremely noticeable. When driving you never notice anything – neither during cold nor warm running. The topic with d The cold start is only recently, with the high speed and the buzzing already the whole time. But that is certainly not normal. Do you have any solutions what it could be, or what would be defective? I look forward to your opinions. LG